This invention relates to radio communications in a receiver and more particularly to a method and system for training an equalizer in a radio receiver.
In recent years, wireless communication systems have been used to convey a variety of information between multiple locations. With digital communications, information is translated into a digital or binary form, referred to as bits, for communications purposes. The transmitter maps this bit stream into a modulated symbol stream, which is detected at the digital receiver and mapped back into bits and information.
In digital wireless communication, the radio environment presents many difficulties that impede successful communication. One difficulty is that the signal level can fade, because the signal may travel in multiple paths due to reflections caused by various objects. As a result, signal images arrive at the receiver antenna out of phase. This type of fading is commonly referred to as Rayleigh fading, fast fading, or multi-path fading. When the signal fades, the signal-to-noise ratio becomes lower, causing a degradation in the communication link quality.
Raleigh fading can be mitigated by using diversity, such as antenna diversity, at the receiver. The signal is received on a plurality of antennas. Because the antennas have slightly different locations and/or antenna patterns, the fading levels on the antennas are different. In the receiver, these multiple antenna signals are combined either before or after signal detection using such techniques as maximal-ratio-combining, equal-gain-combining, and selective combining. Diversity combining techniques are well known to those skilled in the art.
A second problem occurs when the multiple signal paths are much different in length. In this case, time dispersion occurs in which multiple fading signal images arrive at the receiver antenna at different times, giving rise to signal echoes. Between the multiple signals images, the echoes of one symbol interfere with subsequent symbols, causing inter-symbol interference (ISI). As a result of ISI, the bit error rate (BER) of the bit stream cannot be improved below an error floor, also known as the irreducible BER.
Time dispersion affects are further complicated by the presence of multiple frequency components within the message bandwidth. As the frequency differential between frequency components increases, the multi-path signal paths will affect the amplitude and phase of each component differently. As a result, one frequency component may suffer a severe fade while the other frequency component's fade occurs at a different time. This is known as frequency-selective fading.
Interference caused by time dispersion can be mitigated by using an equalizer. In one approach, commonly referred to as adaptive equalization, the equalizer's coefficients are continually and automatically adjusted directly from the transmitted data. A drawback of adaptive equalization is the computational burden involved, since the equalizer must continually update the filter coefficients so that the channel model is adapted to the current conditions of the channel. Equalizer coefficient computational methods employing adaptive algorithms such as the least mean square (LMS) or recursive least square (RLS) are computationally expensive. Communication systems using a high data rate require a very high sampling rate and continual LMS updating of the equalizer filter coefficients, which requires extensive computations.
Common forms of equalization are provided by linear equalizers, decision-feedback equalizers, and maximum-likelihood sequence-estimation (MLSE) equalizers. A linear equalizer compensates for interference in the channel by filtering the received signal. A decision-feedback equalizer exploits previous symbol detections to compensate for the ISI from echoes of these previous symbols. Finally, an MLSE equalizer hypothesizes various transmitted symbol sequences and, with a model of the dispersive channel, determines which hypothesis best fits the received data. Equalization techniques are discussed in further detail in U.S. Pat. No. 5,577,068, which is incorporated by reference.
Radio receivers may use training sequences to adjust equalizer coefficients to compensate for frequency-selective fading. Training sequences are symbol sequences which are inserted by a transmitter at known positions in the transmit symbol stream. The receiver compares a received training sequence with a locally-generated or otherwise known reference training sequence. The reference training sequence used by the receiver is the inserted training sequence prior to experiencing the multi-path channel effects. The receiver determines the differences between the two sequences and uses the determined differences to set the equalizer filter coefficients, since the determined differences correspond to the characteristics of the channel, i.e., the delays and magnitudes of the most prominent echoes.
In practice, different modulation schemes require different training sequences. Preferably, a training sequence is modulated using the same modulation scheme as is used for the information stream to which the equalization is to be applied.
Additional complications arise in communication systems employing dynamic link adaptation. Dynamic link adaptation is a communication method in which the modulation scheme is changed in response to the current link condition. A more robust modulation scheme is employed as needed to compensate for degraded link performance.
Typically, in digital communication, frames or packets are used that are preceded by a header or preamble, which is followed by an information stream, i.e., the payload. The header and/or preamble is typically modulated using a robust modulation scheme to increase the probability of accurate transmission of the header. In order to increase the data transfer rate, the payload is typically modulated using a less robust modulation scheme, which may change depending on the link conditions.
If the received training sequence is positioned within the header, then the received training sequence is transmitted using the more robust modulation scheme of the header, which is more immune to the effects of the multi-path. The payload is typically modulated using a less robust modulation scheme, and therefore experiences more ISI than the header. Consequently, the receiver compares the reference training sequence with a received training sequence that is more immune to the effects of the multi-path, and therefore has experienced less ISI as compared to the payload. As a result, the equalizer receives insufficient information to accurately estimate the channel parameters with respect to the payload, where the equalization is applied.
Ideally, the received training sequence should be modulated using the modulation scheme of the payload, and the reference training sequence should be selected according to the modulation scheme of the payload.
Accordingly, there is a need to provide a method for training a radio receiver in which received training sequences are positioned, and reference training sequences are selected, to accurately reflect the modulation scheme applied to the payload, even where dynamic link adaptation is applied.